From publisher blurb:
Underdark provides a wealth of highly detailed information about one of the most popular regions in the Forgotten Realms setting, the world beneath the ground. This book includesthe following:
- details on the most popular Underdark race, the drow, plus 16 other underground character races;
- 25 new regional feats;
- new prestige classes, spells, monsters, and magic items; plus
background on 60 cities and sites of interest, including content gathered from a multitude of Forgotten Realms products and articles.
To help both players and Dungeon Masters use the book without players stumbling onto things they shouldn't, additional material for running a campaign is isolated in a single section of the book and includes adventure hooks.